Free space optical communication system and method

ABSTRACT

A free-space optical communication method is provided. The method includes generating, at a transmitter of a satellite, an optical frequency comb and a pump signal, modulating the optical frequency comb to generate a data signal and an idler signal that is a phase conjugate of the data signal, attenuating the pump signal, transmitting over free-space, from the satellite, a communication signal having the data signal, the idler signal and the pump signal, receiving from the satellite, at a receiver, the transmitted communication signal having the data signal, the idler signal, and the attenuated pump signal, amplifying, at a phase-sensitive amplifier, the data signal and the idler signal, and demodulating the data signal and the idler signal to extract data.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/856,479, filed Apr. 23, 2020; which is a continuation of U.S. patentapplication Ser. No. 15/390,079 (now U.S. Pat. No. 10,673,530), filedDec. 23, 2016, which claims priority to U.S. Provisional Application No.62/404,316, filed Oct. 5, 2016, the entire disclosures of which are eachhereby incorporated by reference as if set forth in their entiretyherein.

FIELD

The present application generally relates to communication systems andmethods, and particularly to a free-space optical communication systemand method.

BACKGROUND

Interest in free-space communications is increasing, driven by a marketpotential for communications access in places where traditionalcommunication infrastructure (wired or wireless) is limited, or isdifficult to implement (e.g., due to unfavorable geographical terrains).For example, broadband communications for Internet requires transmittinglarge data sets. The current solution for broadband communicationsbetween satellites and between satellites and Earth is radio frequency(RF) communications. However, RF is limited in bandwidth and limitationsare placed on spectrum by government. Further, RF communicationsrequires large beam diameter, and is susceptible to interference,interception, and jamming.

An important parameter associated with implementing conventionalsatellite communications is the size, weight and power (“SWaP”)requirement of the satellite's hardware. For example, it costs USD5000-10,000 to launch a pound of weight in space. One substantialcontributor to the weight of the satellite is the high-power amplifierin the transmitter of the satellite. The high power amplifier is neededto account for losses in transmission of signals from the satellite to aground based receiver.

Various aspects of this application are directed towards addressingthese and other drawbacks and challenges of conventional free-spaceoptical systems and methods with a need to reduce the SWaP requirements.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to limit the scope of theclaimed subject matter. The foregoing needs are met, to a great extent,by the present patent application directed to a system and a method forsecuring a network device. The present patent application will bediscussed in more detail below.

In accordance with an aspect of the patent application, a free-spaceoptical communication method is provided. The method includesgenerating, at a transmitter of a satellite, an optical frequency comband a pump signal. The method includes modulating, at the transmitter,the optical frequency comb to generate a data signal and an idler signalthat is a phase conjugate of the data signal. The method includesattenuating, at the transmitter, the pump signal. The method includestransmitting over free-space, from the satellite, a communication signalhaving the data signal, the idler signal and the pump signal. The methodincludes receiving from the satellite, at a receiver, the transmittedcommunication signal having the data signal, the idler signal, and theattenuated pump signal. The method includes amplifying, at aphase-sensitive amplifier in the receiver, the data signal and the idlersignal. The method includes demodulating, at the receiver, the datasignal and the idler signal to extract data.

In accordance with another aspect of the patent application, atransmitter configured to generate a communication signal for free-spacetransmission is provided. The transmitter includes an optical frequencycomb generator (OFCG) configured to output optical tones. Thetransmitter includes a first array waveguide coupled to an output of theOFCG and configured to split the optical tones. The transmitter includesa pair of modulators coupled to the first array waveguide and configuredto modulate the split optical tones to generate a data signal and anidler signal that is phase conjugated relative to the data signal. Thetransmitter includes a second array waveguide coupled to the pair ofmodulators and to a pump signal generator outputting a pump signal. Thesecond array waveguide is configured to multiplex the data signal, theidler signal and the pump signal to an optical amplifier configured togenerate the communication signal.

In accordance with another aspect of the patent application, a receiverconfigured to receive a communication signal over free-space from asatellite is provided. The receiver includes a wavelength divisiondemultiplexer configured to output a data signal and an idler signal inthe communication signal at a first output of the wavelength divisiondemultiplexer, and configured to output a pump signal at a second outputof the wavelength division demultiplexer, based upon the communicationsignal received over free space from a satellite. The receiver includesa wavelength division multiplexer operatively coupled at a first input,to the wavelength division demultiplexer, and operatively coupled at asecond input, to an output of a frequency stabilizer, the frequencystabilizer being operatively coupled to the wavelength divisiondemultiplexer. The receiver includes a phase sensitive amplifier coupledto the wavelength division multiplexer. The receiver includes feedbackloop coupled to the wavelength division multiplexer in parallel with thephase sensitive amplifier and configured to phase-lock the pump signalwith the data signal and the idler signal. The receiver includes ademodulator coupled to an output of the phase sensitive amplifier andconfigured to recover data in the received communication signal from thedata signal and the idler signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a more robust understanding of the patentapplication, reference is now made to the accompanying drawings, inwhich like elements are referenced with like numerals. These drawingsshould not be construed to limit the scope of the application and arefor illustrative purposes only.

FIG. 1 illustrates an exemplary free-space optical communication system,in accordance with an aspect of this patent application.

FIG. 2 illustrates a first exemplary implementation for a transmitter ina satellite of the free-space optical communication system of FIG. 1 ,in accordance with an aspect of this patent application.

FIG. 3 illustrates an example of optical tones of an optical frequencycomb generator in the transmitter of FIG. 2 , in accordance with anaspect of this patent application.

FIG. 4 illustrates a second exemplary implementation for a transmitterin a satellite of the free-space optical communication system of FIG. 1,

FIG. 5 illustrates a copier-phase sensitive amplifier (PSA) schemeimplemented using the transmitter of FIG. 4 , in accordance with anotheraspect of this patent application.

FIG. 6 illustrates an exemplary implementation of a receiver in aground-station of the free-space optical communication system of FIG. 1, in accordance with an aspect of this patent application.

FIG. 7 illustrates a first transmission method, in accordance with anaspect of this patent application.

FIG. 8 illustrates a second transmission method, in accordance withanother aspect of this patent application.

FIG. 9 illustrates a reception method, in accordance with an aspect ofthis patent application.

DETAILED DESCRIPTION

Various aspects of this patent application (hereinafter “application”)will now be described with reference to the drawing figures, in whichlike reference numerals refer to like parts throughout. A detaileddescription of this application is provided in reference to variousfigures, embodiments and aspects herein. Although this descriptionprovides detailed examples of possible implementations, it should beunderstood that the details are intended to be examples and thus do notlimit the scope of the application.

One or more aspects of this application are directed to systems andmethods for free-space optical communications. The term “free-space” mayapply to optical communications carried out without using optical fibersor optical waveguides as a communication channel. Further, the term“free-space” may include outer space, upper and lower atmosphere, and/orunderwater channels, or combinations thereof, over which communicationsignals are transmitted and/or received without using wires, cables, oroptical fibers between a transmission point and a reception point. Byway of example only and not by way of limitation, such free-spaceoptical communication may be carried out for a space-to-Earthcommunication link where a transmission occurs from one or moresatellites in space and a reception occurs at any point on Earth thatmay or may not fall under a satellite's footprint on the Earth. Still byway of example only and not by way of limitation, the free-space opticalcommunication may be a laser-communication at 1550 nm, 950 nm, and/orother suitable optical wavelengths.

Interest in free-space optical communications, e.g., lasercommunications, from space is exploding, driven by an untapped marketfor Internet access in places that have limited communicationsinfrastructure. Aspects of this application provide a ground-basedreceiver with high sensitivity and an energy efficient transmitter on asatellite to address the size, weight and power reduction challengesfaced by conventional free-space optical communication systems. Aspectsof this application are directed to solving these challenges and otherchallenges, for example, in providing Internet access in underservedareas of the world using laser communications from a satellite. Further,aspects of this application may be utilized, for example, in deep spaceexploration, for battlefield military communications, intelligencegathering, surveillance, reconnaissance, and/or for service and contentproviding.

Referring to FIG. 1 , a general architecture of a free-space opticalcommunication system 100 (interchangeably referred to as “system 100”)is provided, in accordance with an aspect of this application. Thesystem 100 includes a satellite 102 and a plurality of ground-basedreceivers 104(1)-104(n), ‘n’ being an index greater than or equal toone. It will be appreciated that although FIG. 1 illustrates only onesatellite 102, the system 100 may include a plurality of satellites, forexample, a constellation of satellites to which the satellite 102belongs. Further, the satellite 102 may communicate with outer spacebased objects, e.g., other satellites that may or may not be a part of anetwork with which the satellite 102 is associated. The satellite 102may be a low-Earth orbiting satellite or a high-altitude deep spacesatellite (e.g., 700 miles above sea level). Furthermore, instead of orin addition to the satellite 102, the system 100 may include airbornecommunication systems such as those on unmanned or manned aerialvehicles or devices (e.g., drones).

The satellite 102 is configured to transmit one or more laser signals106 to one or more of the ground-based receivers 104(1)-104(n) overfree-space. Such laser signals 106 may each include one or morecommunication signals carrying data. The communication signals are thenprocessed by the ground-based receivers 104(1)-104(n) to recover datafor terrestrial distribution (e.g., over the Internet). The ground-basedreceivers 104(1)-104(n) may be standalone independent ground terminalsor may be connected to each other via one or more communicationnetworks. Further, the one or more of the ground-based receivers104(1)-104(n) may be mobile. The communication signals received by oneor more of the ground-based receivers 104(1)-104(n) may include noise,and may suffer attenuation, dispersion, interference (e.g., due toreflections) and/or other types of distortions (linear as well asnonlinear) due to traversal through the atmosphere, a water body,clouds, or the like, or combinations thereof.

Referring to FIG. 2 , a first exemplary implementation for a transmitter200 in the satellite 102 of the free-space optical communication system100 of FIG. 1 is illustrated, in accordance with an aspect of thispatent application. The transmitter 200 includes a pump signal generator202. The pump signal generator 202 may be a pump laser having acontinuous wave output used as a pump signal “p.” For example, the pumplaser may output a single monochromatic optical signal at a specificwavelength (e.g., 1550 nm). Alternatively, the pump laser output may beused to generate optical signals at two or more different wavelengths,for example using additional optical components and optical setup (e.g.,non-linear components). Still alternatively, a second pump wavelengthcould be created either by two pump lasers or by taking one of the combwavelengths as a pump wavelength.

In one aspect of this application, the pump signal generator 202 mayinclude only a single laser that is configured as a source for alloptical wavelengths in the transmitter 200. Further, this single lasermay be matched to one or more lasers in one or more of the ground-basedreceivers 104(1)-104(n). The same laser in the pump signal generator 202is used to produce both the seed to generate an optical frequency comb300 (shown in FIG. 3 ) and the pump signal “p” for a phase sensitiveamplifier in one or more of the ground-based receivers 104(1)-104(n).

Alternatively, the pump signal generator 202 may provide a basewavelength for outputting two pump wavelengths, which may beadvantageous in mitigation of phase noise introduced by the frequencybroadening needed to reduce stimulated Brillouin scattering (SBS)occurring in the transmitter 200. By way of example only and not by wayof limitation, the pump signal generator 202 may output an optical beamat a wavelength of 1550 nm.

The output from the pump signal generator 202 may be provided to a phasemodulator 204 in the transmitter 200. The phase modulator 204 isconfigured to mitigate the stimulated Brillouin scattering.

The transmitter 200 may include a polarization controller 206 coupled toan output of the phase modulator 204. The polarization controller 206 isconfigured to output the optical beam from the pump signal generator 202to a fixed polarization state. The fixed polarization state of the pumpsignal from the pump signal generator 202 is then outputted to anerbium-doped fiber amplifier (EDFA) 208 in the transmitter 200. The EDFA208 has an output power of 14 dBm (˜2 W of electrical power consumption)to set an input power to a required level for an optical frequency combgenerator (OFCG) 210 in the transmitter 200, the OFCG 210 being coupledto the EDFA 208. In this sense, the OFCG 210 is operatively coupled tothe pump signal generator 202 via the EDFA 208 whose output (amplifiedpump signal) serves as a source for the OFCG 210.

In one aspect of this application, the output from the EDFA 208 may besplit to provide a first portion to the OFCG 210 and a second portion ofthe pump signal to a variable attenuator 212. By way of example only andnot by way of limitation, such a split may be in a 50:50 ratio, or maybe in a 90:10 ratio with 90% of the output of the EDFA 208 beingprovided to the OFCG 210 and the remaining 10% of the pump signal “p” tothe variable attenuator 212.

The OFCG 210 is coupled to the EDFA 208 at an input and to a first arraywaveguide 214 a at an output. In one aspect of this application, theOFCG 210 may be a comb source configured to produce the opticalfrequency comb 300 shown in FIG. 3 (e.g., a 32 nm wide comb), based, forexample, on a Lithium Niobate modulator in a Fabry-Perot cavityarrangement (not shown). The optical frequency comb 300 includes opticaltones 302(1)-302(k), ‘k’ being an integer index greater than or equal totwo, that are phase synchronized as pairs prior to being input to thefirst array waveguide 214 a.

The first array waveguide 214 a is configured to operate upon each of apair of optical tones (e.g., the optical tones 302(1) and 302(2)) inparallel by acting as a router. A first optical tone output by the firstarray waveguide 214 a is modulated by a first modulator 216 a togenerate a data signal (indicated as “s” in FIG. 2 ). In parallel, asecond optical tone output by the first array waveguide 214 a ismodulated by a second modulator 216 b to generate an idler signal(indicated as “i” in FIG. 2 ). The first array waveguide 214 a outputspairs of optical tones (e.g., first and second optical tones 302(1) and302(2)) from the optical frequency comb 300. In one aspect of thisapplication, the data signal “s” and the idler signal “i” may be atfrequencies that are spaced equally from a pump frequency of the pumpsignal “p.”

By way of example only and not by way of limitation, the first modulator216 a and the second modulator 216 b may be in-phase/quadrature phasemodulators (IQ modulators). In one aspect of the application, the idlersignal “i” is a phase conjugate of the data signal “s.” That is, a phaseof the idler signal “i” is shifted by 90° relative to a phase of thedata signal “s.” The first modulator 216 a and the second modulator 216b may each be respectively coupled to a first modulator driver 224 a anda second modulator driver 224 b. Each of the first modulator driver 224a and the second modulator driver 224 b receives data from coding andframing electronics 222 that may be further coupled to a raw datasource, e.g., a database of a computer (not shown) onboard the satellite102. The data modulation and its phase conjugate are transferred via thefirst modulator 216 a and the second modulator 216 b to form the datasignal and the idler signal, respectively. Compound modulation formatssuch as Pulse Position Modulation (PPM) mixed with quadrature phaseshift keying (QPSK) may be enabled using the phase conjugation betweenthe idler signal “i” and the data signal “s.”

The respective outputs (the data signal “s” and the idler signal “i”) ofthe first modulator 216 a and the second modulator 216 b are coupled toa second array waveguide 214 b. In parallel, the second array waveguide214 b receives the pump signal (indicated as “p”) from the pump signalgenerator 202 via the EDFA 208. The second array waveguide 214 b isconfigured to multiplex the data signal “s”, the idler signal “i” andthe pump signal “p” after attenuation by the variable attenuator 212.The multiplexed signal including the data signal “s”, the idler signal“i” and the pump signal “p” is provided to a high-power opticalamplifier (HPOA) 218. The pump signal “p” is at least partiallyattenuated to minimize its effect on a gain of the HPOA 218 for the datasignal “s” and the idler signal “i” wavelengths.

In one aspect of this application, the HPOA 218 is configured to amplifythe multiplexed signal and a communication signal 220 for transmissionby the satellite 102. The communication signal 220 may be transmitted asthe one or more laser signals 106 from the satellite 102 to one or moreof the ground-based receivers 104(1)-104(n).

Exemplary advantages of the transmitter 200 design using the opticalfrequency comb 300 are that the transmitter 200 requires fewercomponents than conventional transmitters since a single laser is usedto produce the data signal “s”, the idler signal “i”, and the pumpsignal “p” via the OFCG 210, uses lesser power, and phasesynchronization between the data signal “s”, the idler signal “i”, andthe pump signal “p” is simplified.

In one aspect, one or more components of the transmitter 200 may beintegrated on a single integrated circuit (IC) chip to reduce size andweight further. For example, an integrated transmitter having afunctionality of the transmitter 200 in a silicon photonics platform maybe implemented. Further by way of example only and not by way oflimitation, the OFCG 210, the first modulator 216 a and the secondmodulator 216 b, and the first array waveguide 214 a and the secondarray waveguide 214 b could all be integrated on a single chip.Furthermore, optical systems integrated on chip in a variety of materialplatforms and on hybrid integration of chips fabricated in differentmaterials may be utilized to implement the transmitter 200, therebyreducing the SWaP of the satellite 102.

Referring now to FIG. 4 , a second exemplary implementation for atransmitter 400 in the satellite 102 of the free-space opticalcommunication system 100 of FIG. 1 is illustrated, in accordance withanother aspect of this patent application. The transmitter 400 uses acopier-phase sensitive amplifier (PSA) scheme shown in FIG. 5 for afree-space communication environment such as that illustrated for thesystem 100 in FIG. 1 . In the copier-PSA scheme for the system 100implemented using the transmitter 400, the transmitter 400 can achieveup to 3 dB noise figure improvements over either a conventionalphase-insensitive erbium amplifier or an all-phase sensitive amplifier.

Referring to FIG. 5 , a schematic diagram of the copier-PSA technique isillustrated for the transmitter 400, in accordance with an aspect ofthis application. A copier 502 is a parametric, phase insensitiveamplifier (an example of which is a length of a highly nonlinear fiber(HNLF)) that creates a phase conjugate idler signal 512 such that a datasignal 508 and the idler signal 512, carrying the same data, are inputto a second stage which is a phase sensitive amplifier 506 after afree-space link loss compensation 504 (e.g., to compensate for the lossbetween the satellite 102 and the ground-based receivers 104(1)-104(n)).The phase conjugate idler signal 512 is what allows this scheme to workwith arbitrary modulation formats as data is encoded in both quadraturesof the data signal 508.

The copier-PSA scheme for the transmitter 400 is uniquely suited to thefree-space optical communications system 100, for which there are notransmission impairments due to dispersion or optical fibernonlinearities, a feature common with the comb based transmitter 200. Inthis scheme, noise is attenuated by the free-space link losscompensation 504 to mitigate correlated noise generated by the copier502 that may dominate the noise figure (NF). It has been proventheoretically that the copier-PSA approach can result in up to a 3 dBreduction in the quantum limit of the NF relative to a conventionalphase insensitive amplifier receiver. The 3 dB value is calculated basedon using the combined signal having the data signal 508 and the idlersignal 512 at an input of the PSA 506. The copier-PSA approach has beendemonstrated to have an NF of 1.1 dB measured at 26.5 dB of gain.Because of the phase conjugated idler signal 512 generated in the copier502, there is no phase squeezing in the phase sensitive amplifier 506 ofthe transmitter 400, and broadband, arbitrarily modulated data can beamplified using the transmitter 400, and transmitted with the pumpsignal 510.

Referring back to FIG. 4 , the transmitter 400 includes a pump laser 402(similar to the pump signal generator 202 in the transmitter 200),coupled to a phase modulator 404 (similar to the phase modulator 204 inthe transmitter 200), a polarization controller 406 (similar to thepolarization controller 206 in the transmitter 200), and an EDFA 408(similar to the EDFA 208 in the transmitter 200) whose output isprovided to a filter 410. The output of the filter 410 is the pumpsignal 510 (shown in FIG. 5 , similar to the pump signal “p” in FIG. 2). An output of the filter 410 is provided to a first wavelengthdivision multiplexer 436.

In parallel, the data signal 508 is provided to the first wavelengthdivision multiplexer 436 in the transmitter 400. The data signal 508 maybe generated using coding and framing electronics 422 (similar to thecoding and framing electronics 222 of the transmitter 200) coupled to amodulator driver 424 (similar to the modulator drivers 224 a/224 b).Further, the modulator driver 424 is coupled to a modulator 416 (similarto the modulators 216 a and 216 b). The modulator 416 may be controlledusing bias control electronics 426, a distributed feedback (DFB) laser428 coupled to a DFB controller 430, and a monitor tap 432. An output ofthe monitor tap 432 is provided to a variable optical amplifier 434 thatis further coupled to the first wavelength division multiplexer 436.

The first wavelength division multiplexer 436 is configured to multiplexthe data signal 508, the idler signal 510, and the pump signal 510, andconfigured to provide them to a parametric amplifier 438 that emulatesthe copier 502. By way of example only, the parametric amplifier 438 mayinclude a highly nonlinear fiber (HNLF) element for amplifying the datasignal 508 and generating the phase conjugate idler signal 512 (e.g.,using four-wave mixing).

The pump laser 402 is an amplified continuous wave (CW) source,requiring phase modulation of the pump signal 508 at the phase modulator404 for mitigation of SBS in the parametric amplifier 438 (e.g., theHNLF). The pump power required at the parametric amplifier 438 isroughly 1 W from the EDFA 408, which has a power consumption of 25 W(˜30 dBm), for example.

An output of the parametric amplifier 438 is provided to a secondwavelength division demultiplexer 440 that splits the data signal 508and the idler signal 512 onto a first fiber 450 and the pump signal 510onto a second fiber 460. The second fiber 460 is coupled to a variableattenuator 412 (similar to the variable attenuator 212 of thetransmitter 200).

An output of the first fiber 450 having the data signal 508 and theidler signal 512, and the second fiber 460 having an attenuated versionof the pump signal 510 are each provided to a third wavelength divisionmultiplexer 442 to multiplex the attenuated pump signal 510 along withthe data signal 508 and the idler signal 512. The pump signal 510 isattenuated by the variable attenuator 412 in order not to rob amplifiergain from the data signal 508 and the idler signal 512.

An output of the third wavelength division multiplexer 442 is providedto a high power optical amplifier 418 (similar to the HPOA 218 of thetransmitter 200). In one aspect of this application, the HPOA 418amplifies the multiplexed signal and to generate a communication signal420 for transmission by the satellite 102. The communication signal 420may be transmitted as the one or more laser signals 106 from thesatellite 102 to one or more of the ground-based receivers104(1)-104(n). The laser signals 106 may be transmitted via telescopeoptics (not shown) in the satellite 102.

The use of a phase sensitive amplifier in one or more of theground-based receivers 104(1)-104(n) is enabled by the creation of thedata signal 508 (or, the data signal “s” in the transmitter 200) and thephase conjugate idler signal 512 (or, the idler signal “i” in thetransmitter 200), e.g., generated by the copier 502. The lower noisefigure of the one or more of the ground-based receivers 104(1)-104(n)enables the required output power from the HPOA 418 (or, the HPOA 218 inthe transmitter 200) to be reduced, thereby reducing size, weight, andpower (SWaP) of the satellite 102. However, in comparison to thetransmitter 200, the transmitter 400 may require a higher number ofcomponents.

Referring to FIG. 6 , an exemplary implementation of a receiver 600 inone of the ground-based receivers 104(1)-104(n) of the free-spaceoptical communication system 100 of FIG. 1 is illustrated, in accordancewith an aspect of this patent application. For example, the receiver 600may be in the ground-based receiver 104(1).

The receiver 600 may include telescopic optics (not shown) to receivethe communication signal 220 and/or the communication signal 420 fromthe satellite 102. Hereinafter, for FIG. 6 , the discussion will referto the communication signal 220 by way of example only and not by way oflimitation. The communication signal 220 is provided to a first splitter602 (e.g., a wavelength division demultiplexer). A first output 604 ofthe first splitter 602 includes the pump signal “p” (or, the pump signal510 as the case might be). A second output 606 of the first splitter 602includes the data signal “s” (or, the data signal 508 as the case mightbe) and the idler signal “i” (or, the idler signal 512 as the case mightbe) combined. In this way, the pump signal “p” associated with the pumplaser of the pump signal generator 202 in the transmitter 200 isseparated from the data signal “s” and the idler signal “i.” The pumpsignal “p” then serves as a signal associated with a master laser fromthe satellite 102.

The first output 604 is provided in succession to a photodiode 614, aloop filter 616, a slave laser 618, a second splitter 620 feeding backinto the photodiode 614, and an amplifier 622 outputting a frequencystabilized pump signal “p” coupled to a piezoelectric transducer 624.The pump signal “p” is frequency locked to the slave laser 618 and thenamplified by the amplifier 622 before being input to a phase sensitiveamplifier 628.

The second output 606 is provided to an optical processor 608. Theoptical processor 608 is configured to adjust or equalize respectiveamplitudes and phases of the data signal “s” and the idler signal “i.”An output of the optical processor 608 is provided to a polarizationcontroller 610 and to a variable delay element 612. The polarizationcontroller 610 is configured to adjust a polarization of the data signal“s” and the idler signal “i” with respect to the pump signal “p.” Thevariable delay element 612 is configured to synchronize optical pathlengths of the data signal “s” and the idler signal “i” with that of thepump signal “p.” An output of the variable delay element 612 is providedto a first input of a wavelength division multiplexer (WDM) 626. Anoutput of the piezoelectric transducer 624 is provided to a second inputof the (WDM) 626. The WDM is configured to combine the data signal “s”,the idler signal “i” and the pump signal “p.”

An output of the WDM 626 is provided to the phase-sensitive amplifier(PSA) 626. By way of example only and not by way of limitation, the PSA628 may be a highly nonlinear fiber amplifier. The PSA 628 is coupled atan output to a filter 630. An output of the filter is provided to acoupler 640. The coupler 640 splits an output of the filter 630 forcoupling onto a WDM 646 and a photodiode 642. Such a split by thecoupler 640 may be, for example, in a 90:10 ratio where 90% of thesignal output by the filter 630 is provided to the WDM 646 and 10% isprovided to the photodiode 642, although this ratio may be adjusted.

The photodiode 642 provides an electrical signal to a lock-in amplifier644. The lock-in amplifier 644 is configured to phase—lock the pumpsignal “p” to the data signal “s” and the idler signal “i.” In thissense, the lock-in amplifier 644 carries out at least a part of the pumprecovery (e.g., carrying out the phase recovery of the pump), the otherpart being carried out using the slave laser 618 where in the pumpsignal “p” or the pump signal 510 is frequency stabilized. Thephase-locked pump signal “p” is then coupled back to the piezoelectrictransducer 624. The photodiode 642, the lock-in amplifier 644 and thepiezoelectric transducer 624 form a feedback loop in the receiver 600.

The WDM 646 has a first output 648 for the data signal “s” and a secondoutput 650 for the idler signal “i.” The data signal “s” is thendemodulated by a first demodulator 652 a coupled at an output to a firstset of balanced photodetectors 654 a and to a first set ofanalog-to-digital (ADC) converters 656 a. Likewise, the idler signal “i”is demodulated by a second demodulator 652 b coupled at an output to asecond set of balanced photodetectors 654 b and to a second set ofanalog-to-digital (ADC) converters 656 b. By way of example only, thefirst demodulator 652 a and the second demodulator 652 b may each be aHybrid IQ demodulator.

An output of the first set of ADCs 656 a is provided to a digital signalprocessor (DSP) 658. Likewise, an output of the second set of ADCs 656 bis provided to the digital signal processor (DSP) 658. The DSP 658 isconfigured to output data packets and a clock signal and forward it toextract raw data and clock by decoding and de-framing electronics 668.Such raw data and clock may then be provided to a computer system (notshown) for use and/or storage.

The receiver 600 works for both transmitter designs for the transmitter200 and the transmitter 400. For communications between the satellite102 and the Earth, a SWaP of the receiver 600 is much less importantthan the SWaP of the transmitter 200 or the transmitter 400. Variousaspects of this application implement a phase sensitive amplifier in thetransmitter 200 or the transmitter 400 with reduced noise figure suchthat a smaller HPOA 218/HPOA 418 could be used in the transmitter200/the transmitter 400, thereby reducing size, weight, and power forthe satellite 102.

As with the transmitter 200 and/or the transmitter 400, the size andweight of the receiver 600 can be reduced dramatically by integration ofthe components on an integrated circuit (IC) chip. For example, thephase sensitive amplifier 628 of the receiver 600 could be integrated ona silicon chip. A nonlinear silicon device could be used instead of theHNLF in the phase sensitive amplifier 628 for the gain medium, and thatcould be integrated with the optical phase lock loop or the feedbackloop in the receiver 600. The high precision of path lengths facilitatedby lithographic definition of optical circuits on an IC chip willsimplify the phase locking of the pump signal “p” (or, the pump signal510) with the data signal “s” and the idler signal “i” (or the datasignal 508 and the idler signal 512).

Phase sensitive amplifiers for which the amplification is based onnon-linear parametric effects, have a quantum noise limit 3 dB lowerthan that of commercial, phase insensitive amplifiers, such as erbiumdoped fiber amplifiers (EDFA). Phase sensitive amplifiers can either befrequency degenerate with identical frequencies for the data signal “s”and the idler signal “i”, or nondegenerate with the data signal “s” andthe idler signal “i” on different frequencies. Parametric amplificationbased on four wave mixing produces side bands at a different (idler)frequency. Degenerate amplifiers, which can be implemented in materialswith high second or third order nonlinearities (χ² or χ³), arecharacterized by lower gain and do not have the flexibility required bya WDM system as only one optical wavelength can be amplified per pumpwavelength. In contrast, non-degenerate phase sensitive amplifiers,which are based on materials with large nonlinearities, for example,highly nonlinear fibers (HNLF) with large χ³ or periodically poledlithium niobate waveguides (PPLN) utilizing cascaded χ² processes, canachieve exponential gain relative to pump power and simultaneousmultichannel amplification. The difficulty is that input signals atdifferent wavelengths must be phase locked. This challenge may bemitigated by electrically modulated sideband generation and parametricidler creation, both of which may avoid the complexity of opticalphase-locking systems.

Referring now to FIGS. 7, 8, and 9 , a free-space optical communicationmethod may be implemented by carrying out a transmission method 700 or atransmission method 800 combined with a reception method 900, inaccordance with an aspect of this application. That is, the receptionmethod 900 may be implemented for either or both of the transmissionmethod 800 and/or the transmission method 900. FIGS. 7, 8, and 9 presentthe transmission method 700, the transmission method 800, and thereception method 900 as flow diagrams, although these may be understoodusing other types of presentations in addition to or as an alternativeto the flowcharts of FIGS. 7, 8, and 9 , such as process and signaldiagrams, graphs, code, charts, equations, timing diagrams, etc. In oneaspect, one or more processes or operations in FIGS. 7 and 8 may becarried out at the transmitter 202 and the transmitter 402,respectively, in the satellite 102. Likewise, one or more processes oroperations in FIG. 9 may be carried out at the receiver 600 in one ormore of the ground-based receivers 104(1)-104(n). The transmissionmethod 700 or the transmission method 800 may at least partially beimplemented by executing the computer executable instructions stored inan internal memory of the satellite 102, which may be executed by aspecial purpose computer having a processor. Likewise, the receptionmethod 900 may at least partially be implemented by executing thecomputer executable instructions stored in an internal memory of theground-based receivers 104(1)-104(n), which may be executed by a specialpurpose computer having a processor.

In yet another aspect, in the transmission method 700, the transmissionmethod 800, and the reception method 900, one or more processes oroperations, or sub-processes thereof, may be skipped or combined as asingle process or operation, and a flow of processes or operations inthe transmission method 700, the transmission method 800, and thereception method 900 may be in any order not limited by the specificorder illustrated in FIGS. 7, 8, and 9 . For example, one or moreprocesses or operations may be moved around in terms of their respectiveorders, or may be carried out in parallel. The term “flow,” as used withrespect to FIGS. 7, 8, and 9 , generally refers to a logical progressionof operations in an exemplary manner carried out at the satellite 102 orthe ground-based receivers 104(1)-104(n). However, such a flow is by wayof example only and not by way of limitation, as at a time, the flow mayproceed along multiple operations or processes of the transmissionmethod 700, the transmission method 800, and/or the reception method900.

The transmission method 700, the transmission method 800, and thereception method 900 may be implemented using a high level or a lowlevel programming language (e.g., C++, assembly language, etc.) usinglogic circuitry (e.g., programmable logic circuit (PLC), etc.) and byexecuting the computer executable instructions. Further, thetransmission method 700, the transmission method 800, and the receptionmethod 900 may be implemented as part of a software application, ahardware implementation, and/or combinations thereof. In one aspect, theinternal memory may include a non-transitory computer readable medium onthe satellite 102 and/or the ground-based receivers 104(1)-104(n). Thenon-transitory computer readable medium may include instructionsthereupon, which when executed by respective processors cause thesatellite 102 and/or the ground-based receivers 104(1)-104(n) toimplement the transmission method 700, the transmission method 800,and/or the reception method 900.

Referring specifically to FIG. 7 , the transmission method 700 may beimplemented at the transmitter 200 of the satellite 102. Thetransmission method 700 may begin in an operation or step 702 where thepump signal “p” may be generated by the pump signal generator 202. Thepump signal “p” may be generated at a pump wavelength by a single laserof the pump signal generator 202. Alternatively, the pump signalgenerator 202 may generate two separate pump signals at two differentwavelengths. From the pump signal “p”, the optical frequency comb 300may be generated by the OFCG 210. To generate the optical frequency comb300, the pump signal “p” is phase modulated by the phase modulator 204and then passed through the polarization controller 206. A part of thepump signal “p” is then split to be sent to the OFCG 210 after suitableamplification (e.g., to ˜14 dBm) by the EDFA 208. In one aspect, theamplification may not be needed in which case passing the pump signal“p” through the EDFA 208 prior to splitting may be optional. The opticalfrequency comb 300 may include equally space optical tones302(1)-302(k), as illustrated in FIG. 4 .

In an operation 704, the optical tones 302(1)-302(k) are split intopairs at the first array waveguide 214 a. A first optical tone (e.g.,the optical tone 302(1)) in the pair of optical tones is provided to thefirst modulator 216 a and a second optical tone (e.g., the optical tone302(2)) is provided to the second modulator 216 b.

In an operation 706, the optical frequency comb 300 is modulated togenerate the data signal “s” at an output of the first modulator 216 aand the idler signal “i” at an output the second modulator 216 b. Morespecifically, pairs of the optical tones 302(1)-302(k) are modulatedrespectively at the first modulator 216 a and the second modulator 216b. For example, the first optical tone 302(1) may be modulated in-phaseusing the modulator driver 224 a that imprints data on the first opticaltone 302(1) and likewise, the second optical tone 302(2) may bemodulated quadrature-phase using the modulator driver 224 b thatimprints data on the second optical tone 302(2), and so on for eachsubsequent pairs of optical tones 302(3)-302(k). In this way, the idlersignal “i” and the data signal “s” are phase conjugated (90° shifted)relative to each other in the IQ modulation scheme implemented at thefirst modulator 216 a and the second modulator 216 b.

In addition, more complex modulation formats such as quadrature-phaseshift keying (QPSK) combined with pulse position modulation (PPM) may beimplemented using the first modulator 216 a and the second modulator 216b. An advantage of using the optical frequency comb 300 is that thetransmitter 200 has fewer components, thus contributing to an overallreduction in the SWaP factors of the satellite 102. Further reductionmay be achieved by integrating the OFCG 210, the first array waveguide214 a, the second array waveguide 214 b, the first modulator 216 a, andthe second modulator 216 b onto an integrated chip (IC), thesecomponents being easier to integrate in an IC than traditionaltransmission components of a conventional satellite.

In an operation 708, in parallel to the modulation of the opticalfrequency comb 300, the pump signal “p” is attenuated by the variableattenuator 212. Such attenuating of the pump signal “p” is carried outto minimize the effect of the pump signal “p” on a gain of the HPOA 218for respective wavelengths of the data signal “s” and the idler signal“i.” The attenuation of the pump signal “p” may be variable and maychange dynamically based on changing conditions for transmission fromthe satellite 102.

In an operation 710, the attenuated pump signal “p” is multiplexed withthe data signal “s” and the idler signal “i” at the second arraywaveguide 214 b. As a result, a multiplexed signal is output at thesecond array waveguide 214 b. In an operation 712, the multiplexedsignal is then provided to the HPOA 218 for amplification. Uponamplification, in an operation 714, the multiplexed signal istransmitted from the satellite 102 as the communication signal 220 toone or more of the ground-based receivers 104(1)-104(n). Thecommunication signal 220 is transmitted as one or more of the laserbeams 106 including the data signal “s”, the idler signal “i”, and thepump signal “p”.

Referring now to FIG. 8 , the transmission method 800 may be implementedat the transmitter 400 of the satellite 102. It will be appreciated byone of ordinary skill in the art that the satellite 102 may at a timehave only one of the transmitter 200 or the transmitter 400 but not bothto keep the SWaP for the satellite 102 low. The transmission method 800may begin in an operation or step 802 where the data signal 508 and thepump signal 510 are generated. In one aspect, the data signal 508 andthe pump signal 510 are generated separately and independently of eachother in parallel. For example, the pump signal 510 is generated by thepump laser 402 and the data signal 508 is generated by the modulator 416configured to modulate a coded and framed data from the coding andframing electronics 422 onboard the satellite 102, which is then passedthrough the variable optical amplifier 434. The data signal 508 and thepump signal 510 are combined by the first WDM 436 in the transmitter400.

In an operation 804, the idler signal 512 is generated from the pumpsignal 510 and the data signal 508. In one aspect, the multiplexedsignal at the output of the first WDM 436 is passed through theparametric amplifier 438 to generate the idler signal 512 from the datasignal 508. In the parametric amplifier 438, SBS effects for the pumpsignal 510 are mitigated. The data signal 508 and the idler signal 512are then separated from the pump signal 510 by the second WDM 440. Thedata signal 508 and the idler signal 512 are passed in the first fiber450 and the pump signal 510 is passed in the second fiber 460.

In an operation 806, the link loss occurring due to the traversal of thecommunication signal 420 is compensated for. For example, the datasignal 508 and the idler signal 512 may be boosted in the HPOA 418 afterthe idler signal 512 has been generated by the parametric amplifier 438.

In an operation 808, the pump signal 510 is attenuated by the variableattenuator 412 in order not to rob amplifier gain from the data signal508 and the idler signal 512. Such attenuation of the pump signal 510 iscarried out prior to the pump signal being provided to the third WDM442, along with the data signal 508 and the idler signal 512. Similar tothe pump signal “p,” the attenuation of the pump signal 510 may bevariable and may change dynamically based on changing conditions fortransmission from the satellite 102.

In an operation 810, the data signal 508 and the idler signal 512 arecombined with the pump signal 510 (after attenuation) at the third WDM442. Then, in an operation 812, the combined signal is amplified by theHPOA 418. Upon amplification, in an operation 814, the combined signalis transmitted from the satellite 102 as the communication signal 420 inthe form of a laser beam.

Referring to FIG. 9 , the reception method 900 is illustrated inaccordance with an aspect of this application. The reception method 900may be combined with either one or both of the transmission method 700and/or the transmission method 900 to form a free-space opticalcommunication method. Further, such a free-space optical communicationmethod occurring between the satellite 102 and one or more of theground-based receivers 104(1)-104(n) may include only parts of thetransmission method 700 and/or the transmission method 800 and thereception method 900. For example, some steps of transmission method 700and/or the transmission method 800, or the reception method 900 may onlybe carried out once when communication between the satellite 102 and theground-based receivers 104(1)-104(n) is initiated, disrupted, orterminated, for example.

The reception method 900 may begin in an operation 902 where one of thecommunication signal 220 and/or the communication signal 420 is receivedby telescopic optics of the receiver 600. For discussion purposes inFIG. 9 , the communication signal 220 will be referred to, although oneof ordinary skill in the art reading this application will appreciatethat this discussion is equally applicable to the communication signal420. Likewise, any processing of the data signal “s” and the idlersignal “i” by the receiver 600 is same as that for the data signal 508and the idler signal 512 generated by the transmitter 400. Accordingly,any discussion with respect to the data signal “s” and the idler signal“i” by the receiver 600 applies equally to the data signal 508 and theidler signal 512 when processed by the receiver 600. The communicationsignal 220 may be received as one of the laser beams 106 at the receiver600 when one of the ground-based receivers 104(1)-104(n), of which thereceiver 600 is part, is directly under a footprint of the satellite102.

In an operation 904, the pump signal “p” is split from the data signal“s” and the idler signal “i” in the received communication signal 220.Such splitting is carried out by the first splitter 602, which may be awavelength division demultiplexer, for example. The pump signal “p” uponsplitting is carried over at the first output 604 of the first splitter602 by a fiber. Likewise, the data signal “s” and the idler signal “i”at the second output 606 are carried over by another fiber to theoptical processor 608.

In an operation 906, the data signal “s” and the idler signal “i” aresynchronized with the pump signal “p” such that data signal “s” and theidler signal “i” have the same optical path length as the pump signal“p.” Such synchronization of the path lengths may be carried out usingthe variable delay element 612.

In parallel with the operation 908, in an operation 908, the pump signal“p” is frequency stabilized using the photodiode 614, the loop filter616, the slave laser 618, and the second splitter 620 feeding back intothe photodiode 614. Such stabilizing of the pump signal “p” includesoutputting the pump signal “p” at the steady frequency of the slavelaser 618.)

In an operation 910, the frequency stabilized pump signal “p” ismultiplexed with the synchronized data signal “s” and the idler signal“i” at the WDM 626. In an operation 912, the multiplexed signal is inputto the PSA 628 for amplification (e.g., using an HNLF). In parallel, inan operation 914, phase-locking of the pump signal “p” is carried out bythe feedback loop formed using the photodiode 642 feeding back the pumpsignal “p” to the WDM 626 via the lock-in amplifier 644 and thepiezoelectric transducer 624. Such phase-locking of the pump signal “p”ensures that a phase of the pump signal “p” is matched with a phase ofthe data signal “s” and is conjugated with the phase of the idler signal“i.”

Finally, in an operation 916, the data signal “s” and the idler signal“i” are demodulated by the receiver 600 to extract data and clocksignals originally transmitted by the satellite 102. Such demodulationmay be carried out by splitting the data signal “s” and the idler signal“i” at the WDM 648 and using the first demodulator 652 a for the datasignal “s” and the second demodulator 652 b for the idler signal “i.”The operation 916 may include individually converting the data signal“s” and the idler signal “i” into equivalent digital signals afterdemodulation for processing by the DSP 658, and decoding and frameremoval by the decoding and de-framing electronics 668.

Advantageously, various aspects of this application make the bus powerdraw of the transmitter 200 and the transmitter 400 lower. By usingmodulation-demodulation techniques in which pulse position modulation iscombined with quadrature phase shift keying, the receiver 600 requiresfewer photons per bit of data in the communication signal 220 or thecommunication signal 420. This can reduce bus power draw by as much as50% for the transmitter 200 and the transmitter 400 leading to a lighterweight for the HPOA 218 and the HPOA 418 in the transmitter 200 and thetransmitter 400, respectively, and hence a smaller SWaP for thesatellite 102. An exemplary comparison of the aspects of thisapplication in terms of bus power draw with respect to conventionalreceivers and transmitters is presented in Table I.

Conventional Phase Receiver 600 Insensitive Amplifier with optical withIntradyne LNA phase-locking Parameter receiver and PSA Noise Figure NF(dB) 3 0 Bus Draw for HPOA 100 W  50 W Preamplifier Bus Draw   2 W¹  25W² ¹Based on EMCORE ® 1014-PA with 14 dBm output, NF = 3.3 dB. ²Based onEMCORE ® 3030 with 30 dBm output, NF ~5 dB.

As understood from Table I above, instead of using a low-noise amplifier(LNA), as is conventional in receivers on ground, this applicationintroduces use of the PSA 628 having a quantum limit of 0 dB on noisefigure in the receiver 600, 3 dB lower than phase insensitiveamplifiers. This 3 dB improvement in noise figure translates directly toa drop in the power required from the transmitter 200 and/or thetransmitter 400.

The present description is for illustrative purposes only, and shouldnot be construed to narrow the breadth of the present patent applicationin any way. Thus, those skilled in the art will appreciate that variousmodifications might be made to the presently disclosed embodimentswithout departing from the full and fair scope and spirit of the presentapplication. Other aspects, features and advantages will be apparentupon an examination of the attached drawings and appended claims.

What is claimed is:
 1. A device comprising: a wavelength divisiondemultiplexer configured to receive a communication signal received fromfree space and output a first signal and a second signal that is a phaseconjugate of the first signal at a first output of the wavelengthdivision demultiplexer, and configured to output a pump signal at asecond output of the wavelength division demultiplexer, based upon thecommunication signal received over free space from a satellite; awavelength division multiplexer operatively coupled at a first input, tothe wavelength division demultiplexer, and operatively coupled at asecond input, to an output of a frequency stabilizer, the frequencystabilizer being operatively coupled to the wavelength divisiondemultiplexer; an optical processor coupled to the output of thewavelength division demultiplexer; a variable delay element coupled tothe optical processor and to the first input of the wavelength divisionmultiplexer, wherein the optical processor and the variable delayelement are configured to pass the first signal and the second signalupon being output by the wavelength division demultiplexer, and areconfigured to synchronize path lengths of the first signal and thesecond signal with the pump signal; a phase sensitive amplifier coupledto the wavelength division multiplexer; a feedback loop coupled to thewavelength division multiplexer in parallel with the phase sensitiveamplifier and configured to phase-lock the pump signal with the firstsignal and the second signal; and a demodulator coupled to an output ofthe phase sensitive amplifier and configured to recover data in thereceived communication signal from one or more of the first signal orthe second signal.
 2. The device of claim 1, wherein the phase sensitiveamplifier includes a highly nonlinear fiber through which the firstsignal, the second signal and the phase-locked pump signal are passed.3. The device of claim 1, wherein the feedback loop comprises aphotodiode coupled to an output of the phase sensitive amplifier and alock-in amplifier coupled to an output of the photodiode.
 4. The deviceof claim 3, wherein the output of the lock-in amplifier is coupled tothe output of the frequency stabilizer.
 5. The device of claim 1,wherein the frequency stabilizer includes a slave laser and an amplifierfor the pump signal at the second output of the wavelength divisiondemultiplexer.
 6. The device of claim 1, wherein the device isconfigured to receive a communication signal over free-space from asatellite.
 7. A system comprising: a first device configured to output acommunication signal via free space; and a second device comprising: awavelength division demultiplexer configured to receive thecommunication signal received from free space and output a first signaland a second signal that is a phase conjugate of the first signal at afirst output of the wavelength division demultiplexer, and configured tooutput a pump signal at a second output of the wavelength divisiondemultiplexer, based upon the communication signal received over freespace from a satellite; a wavelength division multiplexer operativelycoupled at a first input, to the wavelength division demultiplexer, andoperatively coupled at a second input, to an output of a frequencystabilizer, the frequency stabilizer being operatively coupled to thewavelength division demultiplexer; an optical processor coupled to theoutput of the wavelength division demultiplexer; a variable delayelement coupled to the optical processor and to the first input of thewavelength division multiplexer, wherein the optical processor and thevariable delay element are configured to pass the first signal and thesecond signal upon being output by the wavelength divisiondemultiplexer, and are configured to synchronize path lengths of thefirst signal and the second signal with the pump signal; a phasesensitive amplifier coupled to the wavelength division multiplexer; afeedback loop coupled to the wavelength division multiplexer in parallelwith the phase sensitive amplifier and configured to phase-lock the pumpsignal with the first signal and the second signal; and a demodulatorcoupled to an output of the phase sensitive amplifier and configured torecover data in the received communication signal from one or more ofthe first signal or the second signal.
 8. The system of claim 7, whereinthe phase sensitive amplifier includes a highly nonlinear fiber throughwhich the first signal, the second signal and the phase-locked pumpsignal are passed.
 9. The system of claim 7, wherein the feedback loopcomprises a photodiode coupled to an output of the phase sensitiveamplifier and a lock-in amplifier coupled to an output of thephotodiode.
 10. The system of claim 9, wherein the output of the lock-inamplifier is coupled to the output of the frequency stabilizer.
 11. Thesystem of claim 7, wherein the frequency stabilizer includes a slavelaser and an amplifier for the pump signal at the second output of thewavelength division demultiplexer.
 12. The system of claim 7, whereinthe second device is configured to receive a communication signal overfree-space from a satellite.